This invention related to a heated tooling system, in particular to a heated tooling system for increased control of material properties of an article made in said tooling system.
Many materials are moulded using a variety of moulding tools. It is well known in some industries, for example in the metal casting industry, that the rate and temperature at which a material solidifies affects the material properties of the article. This effect usually takes place on a microscopic scale and may for example include such characteristics as the proportion of amorphosity or crystallinity in the final product. When materials such as metals are used, the article can be treated after it comes off the tool or mould, for example by heat treating and quenching, to control the material properties, however these processes are lengthy and increase production time and cost.
When polymer materials are moulded, in particular thermosetting resins, which may also contain a filler, the finished article can not usually be treated by a secondary process to control its structural properties after it comes off the tool. Some tools may be maintained in a temperature controlled environment, for example an autoclave, to regulate the overall tool surface temperature and this can be effective for articles with a relatively thin and constant cross sectional area.
When using tools to produce articles that have a thick or, in particular, a variable thickness the problems associated with variance in material properties of the finished product are further compounded by variance in heat loss from the different thickness sections.
When thermosetting resins are used further complications arise by virtue of the exothermic nature of the process of curing/cross linking. The temperature of the curing thermoset polymer is a product of not only the tool temperature, but also of the local reaction rate and the ability for heat to escape from the area in which the reaction is occurring. The result of this is that the thermosetting polymer will cure at different rates at different points across its profile resulting in different material properties derived from the differing molecular structure.
Metals and thermoplastic polymers will experience different crystallinities in areas of different thickness as the cooling rate will vary dependant on heat extraction. Autoclaves can be used to control this but, in order to overcome the differences due to different cooling speeds of thick and thin parts of an article generally very gradual cooling is used which increases production time. Furthermore autoclaves are notoriously energy inefficient and require large areas to accommodate their footprint which is typically much larger then the actual tool for the article.
Even though the different material properties arising from current tooling systems may, to some extent, be predictable it is not controllable. In many instances, the design of articles, in particular those to be moulded out of thermosetting polymers, is compromised between an ideal design and a design that is practical to process. In particular where an ideal solution for a part would comprise a single large article having sections of different thickness, several smaller articles, each having a more uniform thickness, may be manufactured and assembled together so as to obtain better control over the material properties of each section of the article.
A further problem with current moulding techniques, and particularly associated with autoclaves is that they are not controllable so as to vary the material properties of the articles they produce. For example if a thermoplastic, e.g. PEEK, part of varying thickness were required that had thicker sections required to have a high crystallinity to impart strength and thinner sections having a lower crystallinity to impart flexibility, then current technologies are not effective in creating such a part in a single moulding process. The present invention at least partially mitigates the above mentioned problems with known tooling processes.